In recent years, worldwide restrictions on exhaust gas are becoming tighter from the viewpoint of environmental protection. Exhaust gas purification catalysts are being employed in internal combustion engines as one measure. In order to efficiently remove the hydrocarbons (hereunder also abbreviated as “HC”), CO and nitrogen oxides (hereunder also abbreviated as “NOx”) in exhaust gas, exhaust gas purification catalysts employ a variety of catalysts, including platinum-group elements such as Pt, Pd and Rh, etc., as catalyst components.
Precious metal catalysts known in the prior art can decompose HC, CO and NOx near stoichiometry, but they are all associated with problems of resource depletion, and there is a demand for catalysts using other metals while exhibiting purification performance at or above that of conventional precious metal catalysts, or purification catalysts that can reduce the usage of precious metals.
PTL 1 describes a method in which nitrogen oxides in exhaust gas are reduced to nitrogen by being passed through a catalyst system comprising at least two catalyst beds in the presence of a reducing agent, wherein the first catalyst bed is iron-beta-zeolite and the second catalyst bed downstream from it is silver supported on alumina.
Reference 2 describes a saddle riding vehicle comprising an internal combustion engine that performs combustion at a lower air/fuel ratio than the theoretical air/fuel ratio, a first catalyst provided in an exhaust channel through which combustion gas is discharged from the internal combustion engine, a second catalyst provided at the downstream end from the first catalyst in the exhaust channel, and a secondary air injector that injects secondary air into a section of the exhaust channel between the first catalyst and the second catalyst, wherein the first catalyst and the second catalyst each contain a precious metal component including at least one of Pt, Rh, Pd and Au, and the first catalyst further contains a zeolite carrier and Co or Fe supported on the zeolite support in greater abundance than the ion-exchangeable amount.
Reference 3 describes an exhaust gas purification device for an internal combustion engine comprising a first catalyst device that is situated in the exhaust channel of an internal combustion engine and purifies noxious components in exhaust gas, a first exhaust concentration sensor situated in the exhaust channel at the upstream end from the first catalyst device, having output characteristics that are approximately proportional to the exhaust gas concentration, operation state detection means that detects the state of operation of the engine, including at least the engine rotational speed and engine load condition, first target air/fuel ratio calculation means that calculates the target air/fuel ratio based on the detection results from the operation state detection means, a second exhaust concentration sensor situated in the exhaust channel at the downstream end from the first catalyst device, in which the output signal is inverted near the target air/fuel ratio, enriching means that sets the target air/fuel ratio to a slightly richer state than the theoretical air/fuel ratio based on the output value of the second exhaust concentration sensor, and control means that accomplishes feedback control of the air/fuel ratio of the gaseous mixture detected by the first exhaust concentration sensor, to the target air/fuel ratio set by the enriching means, wherein a second catalyst device is situated in the exhaust channel at the downstream end from the second exhaust concentration sensor, while secondary air supply means is provided which supplies secondary air into the exhaust channel at the upstream end from the second catalyst device and the downstream end from the second exhaust concentration sensor.
Reference 4 describes a gold alloy catalyst composed of Au and one or more elements selected from among the following metals M, the weight ratio of Au and the metals M being Au/M=1/9 to 9/1, and the Au solid solution content in the alloy being 20 to 80 wt % (M: Pt, Pd, Ag, Cu and Ni (claim 1 of Reference 4)).